Support context argument in batch and unbatch methods

[discobot]

What does this PR do?

Fixes #617.

LitAPI.batch and LitAPI.unbatch are the only hooks that never receive the per-request context. It is worse than a missing option: declaring batch(self, inputs, context) with max_batch_size > 1 currently fails every batched request with TypeError: batch() missing 1 required positional argument: 'context', surfaced as HTTP 500.

Root cause: the batched loops (regular and streaming) call lit_api.batch/lit_api.unbatch directly instead of through the _inject_context helper that decode_request/predict/encode_response already go through — even though the contexts list is in scope at all four call sites. This PR wraps those call sites with _inject_context(contexts, ...) and documents the optional context argument in the batch/unbatch docstrings. _inject_context only passes context when the method signature declares it, so existing implementations are unaffected.

Added test_inject_context_in_batch_and_unbatch covering both loops: batch stores each request's input in its per-request context and unbatch reads it back, so the test fails with the TypeError above without the fix. tests/unit/test_lit_server.py, tests/unit/test_batch.py and tests/unit/test_loops.py pass locally.

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